This invention relates to polyvinyl chloride compositions. More particularly, it relates to cross-linkable or curable compositions in which the cross-link promoters are unsaturated derivatives of acrylic or methacrylic acid which have not been previously employed for this purpose. The invention is particularly applicable to the preparation of wire coatings, and flexible and rigid forms such as pipes and rods which may be formed by extrusion or light machine parts and housings which are normally molded.
Polyvinyl chloride is a well characterized and widely employed polymer. It has been used in a large variety of commercial applications, and continues to be so utilized. It has, for example, been molded to form coverings and housings for numerous articles, including telephones, vacuum cleaners, electrical motors and the like. It has been used in wire coatings, and as a substitute for metallic pipe in those instances where it will not be exposed to prolonged or excessive heat.
The properties of polyvinyl chloride can be improved by curing. Such improved properties include better tensile strength, higher modulus at elevated temperatures, higher resistance to creep, flow and distortion at elevated temperatures and better abrasion resistance. More specifically, when polyvinyl chloride is employed as a wire coating, curing improves hot cut through resistance, imparts better resistance to damage caused by contact with hot solder or with the solder iron, and permits the use of relatively thinner coatings because of improved abrasion resistance and strength. With pipes, the improvements include better burst strength at elevated temperature, better resistance to expansion and distortion under load, and better solvent and chemical resistance.
Curing is effected by the formation of cross-links between adjacent polymer molecules. Cross-linking is normally induced by the presence of a free radical generator such as an azo compound or a peroxide.
Attempts have been made to induce cross-linking utilizing high energy radiation. It was found, however, that polyvinyl chloride degrades at a faster rate than it cross-links when subjected to radiation energy at a sufficiently high level to induce cross-linking. To avoid this difficulty, the art has relied more and more on so-called cross-link promoters, both direct and indirect. Direct cross-link promoters are unsaturated compounds such as triallyl cyanurate, triallyl isocyanurate, trimethylolpropane trimethacrylate, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate and diallyl phthalate which under the influence of radiation energy directly enter into the structure of the final molecule. Indirect promoters normally are halogenated compounds such as carbon tetrachloride which do not appear to become a segment of the final cross-linked molecules. In any event, the function of the promoters is to permit the curing of polyvinyl chloride compositions at a sufficiently low total radiation dose so that the polyvinyl chloride cures faster than it degrades.
The presently known cross-linking agents are not completely satisfactory for a number of reasons. For example, those cross-linkers which include an allyl group such as triallyl cyanurate, while an improvement, still require undesirably high energy levels for the cross-linking reaction. Often there is a loss of impact resistance in the final product.
The products produced by radiation cross-linking often manifest undesirably low thermal and heat aging stability. This has been attributed to electron irradiation damage during cross-linking due to high energy levels required even when cross-link promoters are employed. The art has attempted to overcome this problem by the addition of energy absorbers to protect the polyvinyl chloride from radiation damage. Unfortunately, the use of energy absorbers has been found to decrease the rate of cross-linking. This is a serious problem in modern mass production.
Novel polyvinyl chloride curable compositions which substantially alleviate the problems of the prior art and include a class of cross-link promoters not previously described for such use have now been discovered. The curing of these compositions makes possible the production of new products such as insulating wire coatings, pipe and other moldings which are characterized by improved properties, particularly thermal stability, heat aging stability, impact strnegth and resistance to water absorbtion at high temperatures.
The term "polyvinyl chloride" as used in the description of this invention refers to homopolymers and copolymers prepared from vinyl chloride alone or together with other comonomers, particularly .alpha.-olefins such as ethylene and propylene, vinyl ethers and vinyl esters, especially ethers containing up to six carbon atoms, such as methyl, ethyl, propyl and butyl ethers and vinyl acetate, vinyl propionate and vinyl butyrate, as well as acrylic and methacrylic acid derivatives, especially esters such as methyl or ethyl acrylate or the corresponding methacrylates.